System and apparatus for secret radio communication



Dec. 29, 1931. A M JR 1,838,762

SYSTEM AND APPARATUS FOR SECRET RADIO COMMUNICATION Filed March 1'7, 1928 2 Sheets-Sheet 1 a (BM) JOHN HAYS HAMMOND JR.

ORNEY Dec. 29, 1931. J. H. HAMMOND, JR 1,838,762

SYSTEM AND APPARATUS FOR SECRET RADIO COMMUNICATION 2 Sheets-Sheet 2 Filed March 17 1928 Patented Dec. 29, 1931 entree stares JOHN HAYS HAMMOND, 33., OF GLOUCESTER;MASSACHUSETTS SYSTEM AND APPARATUS FOR SECRET RADIO COMMUNICATION Application filed March 17, .i9as. Serial to. 262,365.

ulated by an audio frequency. This system has come to be known as the double modulation system.

An object of this invention is to transmit messages of such a type that reception is impossible for an unauthorized listener not equipped with apparatus constructed in accordance with this invention.

It is a further object of this invention to simplify the apparatus and method necessary for secret radio communication.

Other objects of this invention will become apparent from the following specification taken in connection with the drawings.

Previously apparatus has been designed for secret radio communication which requires the use of some frequency shifting device requiring exact synchronism between the transmitter and the receiving apparatus. This type of apparatus is open to very serious objections in that exact synchronism is not only seldom possible but nearly always impossible.

It is an object of this invention to obviate the necessity for synchronism between receiver and transmitter and-yet prevent detection and reception of the signals transmitted in accordance with my method, by an unauthorized listener.

In accomplishing these objects, I have utilized a B frequency which has been designated to represent a secondary or supersonic lower frequency such as for example of the order of kilocycles. This B frequency is multiplied by a distortion amplifier to produce a carrier frequency of say 500 kilocycles. Both the B frequency and the carrier frequency thus produced are wobbled the small amount of a few cycles per second by the expedient of wobbling the B frequency before production of the carrier frequency. A pushpull modulator is used for modulating the B7 frequency, which has been wobbledyby the audio frequency or A frequency. This will cause a suppression of the intermediate or B frequency and produce inl-the output the bands produced in the first modulationin a "5'5 similar push-pull modulator which is, therefore, adapted to modulate the carrierfrequency by the said side band and produce therefrom two additional side bandsbutsuppress the carrier frequency. Oneof theside bands produced by this latter modulation is transmitted as well as the B frequency. Transmission of these two frequencies is accomplished over separate antennae.

Thus it can be seen that transmission is accomplished over two very narrow frequency bands. I 7

'Reception is accomplished through a special receiver in which the B? frequency is ree ceived distorted and multiplied to provide 7 the carrier frequency. The side band transmitted is received, combined with the carrier frequency produced and again combined with the original B frequency transmitted and thus produces the audio frequency; Having now briefly described my invention attention is invited to the accompanying drawings in which Fig. 1 illustrates the trans mitter adapted to carry out the principles of my invention. Fig. 2 represents the relative S0 locationiof the variousfr-equencies bands in volved in my transmission. Fig. 3 represents the receiver adapted to receive the transmission of a transmitter as shownini Fig.1. Referring now more particularly to Fig. 35 1, there isdia'grammatically shownat 10 a thermionic oscillator tube having filament 11, grid Y12 and plate '13, the former of'which is adapted to be supplied by a current from the source 14, regulated in the usual and wellknown manner. Thegrid and plate circuit of the thermionic device 10 areinterrelated through the winding 16 a central point of which is connected to the filament lead and the plate circuit includes the high potential source 15. Shunted across the inductance 16 is the condenser 17 for determining the frequency of the oscillator and in parallel with said condenser 17 is a variable condenser o 18 which is adapted to be varied cyclically by no the motor 19 connected to the lighting current or other suitable source of power and thus wobble the frequency generated at 10 from B to B-tlv. The output of the oscillator 10 is supplied through the biasing battery 20 to the grid 12 of the amplifier tube 21 included in the plate circuit of which is the high potential source 22. In series with the source 22 is the inductance'23 ofthe coupling 23-25.

The inductance 25 is included -in the antenna circuit comprising said inductance, the ground 27 and the'antenna 26. A third inductance 28 is also inductively related to the inductances 23 and 25, the'use for which will be later pointed out. p I v The output of the amplifier 21 is fed through the filter 24 to the output transfo'rmer 2930 in the grid return circuit of the push-pull modulator comprising the thermionic devices 38 and 39. The audio frequency source or microphone 31 taken in connection'with the potential source 32 is adapted to vary the current flowing through the primary circuit 33 of the transformer 34 in accordance with voice frequencies and consequently vary the current flowingin the sec ondary 35 in such a manner that charges are impressed inverselyupon the grids 12 01 the devices 38 and 39 respectively. Condensers '36 and 37 are connected across the halves of the secondary'35 as shown for the purpose of tuning the input circuits to the desired frequency. The output circuit ofthe pushpull amplifier comprises the primary winding 41 and the high potential source 40.

Inductively coupled to the primary 41 is the secondary 42 the midpoint 43 of which is connected through the inductance54 constituting the secondary of the coupling 53-54 to the filaments of the push-pull modulator comprising the thermionic devices 55 and 56. 'Across the halves of thesecondary 42 are the condensers 44 and 45 for tuning the input circuits to the desired frequency. 1

The thermionic device 46 is adapted to have impressed upon its input or grid 12 va riations of a frequency generated by the device 10 and is overloaded in such a manner that it will act as a distortion amplifier or harmonicgenerator; The high potential source 51 and the choke are shunted by the condenser 47 which is adapted to control the frequency produced in the output or plate circuit which includes also the plate 13 and the primary 48. Inductively coupled to the primary winding 48 is a secondary winding 49 adapted to supply the carrier frequency to the modulators and 56 through the filter 52 and a primary 53 inductively coupled to the secondary 54 as above described.

. The output circuit of the push-pull modulators 55 56 is completed through the primary winding 58 of the transformer 59 and the high potential source 57 in the usual and well-known manner.

Inductively coupled to the primary 58 of the transformer 59 is a secondary 60 included in the antenna circuit which comprises in addition the condenser 61, the antenna 62, and the ground connection 63.

li-eferring'now more particularly Fig. 2 there is shown at 1 a graphic representation of the relative frequency produced by the generator 10 and at 2 a graphic representation of the frequency produced by the harmonicgenerator .46. At 3 and 4 are shown the upper and lower side bands respectively produced by the modulation of the carrier frequency. by the B frequency produced by the generator 10 after modulation in the push-pull modulator 38'39 by the A frequency supplied through the microphone 31.

5 and'6 represent graphically the location of the side bands produced in the push-pull modulator comprising the devices 55 and 56 by the modulation of the carrier frequency supplied by the thermionic device 46 by the side band transmitted through the transformer 4142 in the input of said push-pull modulator device.

It will be noted that all of the frequencies except 1 and 6 indicated on this diagram are in dotted lines which represent that they are not transmitted, the only frequencies being transmitted are B+W representing the sec ondary carrier frequency plus the amount of wobble deviation and the side band produced by the modulation of the carrier frequency 10 (Bi-WV) by the side band produced by the modulation of B by "the audio frequency or 11 (B-l-W) +A.

Referring now more particularly to Fig. 3 there is shown at 110 an antenna the circuit of which includes the condenser 111, inductance 113 andground 114. This antenna circuit is adapted to receive the intermediate carrier frequency B+W and transfer it by means of the inductance 113 coupled to the secondary inductance 115, to the oscillatory circuit including the said inductance and the condenser 116. The condenser 116 is adapted to tune this circuit to 'resonance and supply to the grid 13 of the device 117 the frequency B+VV. The device 117 includes the filament 11 the grid 12 and the plate 13, the filament of which is adapted to be supplied in the usual manner by a current of low potential by the source 118. The grid return is completed through the inductance 119.

The output of the amplifier device 117 includes the resonant circuit 121 and the high potential source120. The resonant circuit 121 is adapted to be tuned to the intermediate carrier frequency B+W and includes the condenser 122 and the inductance 123 which latter comprises the primary of the coupling 123 124 the latter of which is connected to the grid 12 of the amplifier device 125. The input or grid circuit of the device 125 includes a source of low potential 127 for duct'ance 131 is inductively coupled to a secondary inductance 132 which is adapted to supply the grid 12 of the thermionic device 133 with the frecuenc of the secondar carrier B+W. Included in the grid circuit of the device 133 is a biasing battery 135 which isadapted to bias the grid of said device so that it will produce a harmonic in its output circuit. The filament 11 is adapted to be supplied in the usual and well-known mar nor by the low potential source 131.

The output of the device 133, which functions as a harmonic generator, includes the tunedcircuit137 and the high potential source 136. The circuit 137 is tuned to a frequency which is the same as the carrier frequency produced by the harmonic generator 16 of Fig. 1.- his circuit includes the inductance 139 and the condenser 138 the former of which is inductively coupled to the input 110 of the filter 111. The filters 21, 52 and 111 are of the band pass types as disclosed in the patent to Campbell dated'May 22nd, 1917, No. 1,227,113. The output of the filter 111 includes the inductance 112.

The antenna 113 includes in its circuitthe capacity 115, the inductance 116 and the ground connection 117 and is adapted to receive the side band being transmitted by the antenna 62 of circuit 1. The inductance in said antenna circuit is inductively coupled to the inductance 118 which with the condenser 119 comprises a closed oscillatory circuit.

The said, oscillatory circuit is adapted to supply through the inductance 150,the grid 12 ofthe thermionic device 151 with the alternating potential corresponding to the side band frequency transmitted from the antenna 62 of Fig. 1. Completing the grid cathode circuit is the tap 158 of the potentiometer 159 connected across the filament circuit for supplying the filament 11. The low potential source 156 is adapted to supply the'said filament of the device 151 through rheostat 157 in the usual and well-knoWn-manner. Included in the input to the grid element 12 of the device 151 is the inductance 150 inductively coupled to the output inductance 112 of the filter 111.

The output circuit of the device 151 includes the oscillatory circuit 160 and the high potential source 163. The oscillatorycircuit 160 comprises the capacity 161 and the inductance 162 which former is adapted to adjust the frequency of the oscillatory circuit W 0 160 to the secondary carrier frequency plus the audio frequency which is one of the side bands produced by the combining of the carrier frequency as produced by the distortion amplifier 133 and supplied through the filter 111 to the input of said device 151 and the side band transmitted through the antenna circuit 113117.

Inductively coupled to the inductance 162 I is the inductance 161 of the closedioscill'atory circuit comprising, in addition to the inductance 161, the capacity 165.- This circuitis likewise tuned to the side band produced by the device 151 and is connected through the inductance 171 to the grid 12 of the device 172; the oscillatory circuit 161-165 is connected to the filament 11 of the device 172 by means of the tap 166 of the potentiometer 173 which is connected across the filament leads of the said device 17 2. The source 156 supplies the filament 11 of the device 172 with the necessary current which is adjusted by means of the rheostat 171.

The inductance 119 is included in the grid circuit of'the device 117 and is inductively coupled to the inductance 167 which in turn is connected to the input of device 168. The device 168 amplifies the frequency impressed upon the grid of said device and produces in the output, including the inductance 169 and the high potential source 170, a frequency which is the same as the secondary carrier frequency transmittedby the antenna-26 of Fig. 1. Inductively coupled to the inductance 169 is the inductance 171 included in the grid circuit of the device 172.

The output of the amplifier 17 2 includes the telephones 17 5 or other suitable recording or indicating instrument, and the high potential source 163, the former of which'may be shunted by the condenser 176 as shown.

Having now described the apparatus for carrying out my invention, I will briefl descr1be 1ts operation. I

the side bands B+W-PA and B+W-A.

Inductively coupled to the output of this push-pull modulator is the input circuit of a.

second push-pull modulator comprising the put of said modulator as .B+i V+A, and 21 (Ed-v. A.

to produce the frequencies 2 thermionic devices and 56. The input circuit of this last mentioned thermionic push-pull modulator is adapted to be tuned to oneof the side bands produced in the first push-pull modulator as for instance, the frequency B+VV+A. In the distortion amplifier or harmonic generator 36 a frequency B+1V produced in the generator 10 is adapted to be multiplied to a frequency such as 10 (B+W).

This harmonic frequency 10 (B+W) is supplied as the carrier frequency to the second "push-pull modulator and is adapted to be modulated in the usual. and well-known manner by the frequency existing in the infor instance B+'W+A. The product of this modulation which suppresses the frequency 10 (Bid/V) is the side bands 10 (B-FW) (Bd-Wd-A) or 11 (B-FWV) +A and 10 (B+W)-(B+ The output circuit of the antenna 62 is adapted to radiate one of the side bandsto the exclusion of the other. In this instance the side band repre sented as 11 (Ed-1V) +A has been transmitted. The original secondary carrier fro quency or B +i/V is adapted to be transmitted also by the antenna 26, the circuit of which is tuned to resonance with the said frequency There thus exist in the ether, two frequencies of 13+? and 11 (Bi-VJ) +1 1, which, being properly combined in the receiver of my invention as illustrated in Fig. 3, will produce the originalaudio frequency.

The secondary carrier frequency B-i-VV ispicked up by the antenna and amplified through the amplifiers 117 and and supplied to the distortion amplifier or harmonic generator 133, after which the frequency is filtered through the filter i il and supplied to the input of the thermionic detector 151. The antenna 153 is adapted to pick up the frequency 11 (B +VJ) +A and supply this frequency to the input of the detector device lei. There is thus present in the input circuit of the device 151 the two frequencies 10 (Bi-Vi?) and 11 (B-l-l V) +A. This detector is so adjusted that in the output circuit there will existthe frequencies The output circuit of the device 151 is tuned to the former of these frequencies, or Bd-VV-Hr. The input of the thermionic detector 172 is adapted to be supplied from the first detector device with the frequency B-l-VV-i-A and also through the input of the secondary carrier frequency receiving circuit with the frequency B+VV through the amplifier 168. In this device 17 these two frequencies combine (Ed-1V) +A and A, the latter of which is adapted to actuate the receiver 175 and produce therein the original or audio frequency. 7 V

I have described above but one modification of my invention. Other modifications u I n to be transmltted, modulating sa1d wobbled low supersonic frequency wave by said audio irequency current, suppressing the low supersonic frequency wave, and one of the side bands pro by said modulation, producin'" a u-monic of said low wobbled superouency W and providing thereby d carrier frequency wave, filtering said wobbled carrier frequency wave, modulating said our frequency wave by the reide band from said first mentioned moduia n, suppressing the Wobbled carrier frequency wave and one of the side bands iced by said hast mentioned modulatransmitting the remaining side band low wobbled supersonic frequency Wave transmitted, amplifying and producing from said wohbled frequency wave the Wobbled carrier frequency wave, receiving the side band transmitted, combining said side band and said wobbled carrier frequency wave and detecting to produce said low wobbled supersonic side band, and combining said last mentioned side band with said transmitted low wobbled supersonic frequency ware and detecting to produce said audio frequency current.

2. The method of secret radio communication which includes generating a low supersonic frequency wave, wobbling the frequency of said wave, transmitting said wobbled low supersonic frequency wave, filtering said wobbled supersonic frequency wave, producing an audio frequency current to be transmitted, modulating said Wobbled low supersonic frequency wave by said audio frequency current, suppressing the low supersonic frequency wave and one of the side bands produced by said modulation, producing a harmonic of said wobbled supersonic frequency wave and providing t ercby a Wobbled carrier frequency wave, filtering said Wobbled carrier frequency wave, modulating said carrier frequency wave by the remaining side band from said first mentioned modulation, suppressing the wobbled carrier frequency wave and one of the side bands produced by said modulation, and transmitting the remaining side band produced by said modulation.

3. 'The method of secret radio communica- ,tion which comprises receiving a 10W Wobbled supersonic frequency Wave, amplifying and producing from said Wobbled frequency wave, a Wobbledcarrier'frequency Wave, receiving a side band, combiningisaid side band and said obbledcarricr frequency Ware and detecting to produce a low Wobbled supersonic side band, combining said last mentioned side band Withsaid transmitted low Wobbled supersonic-frequency wave. and detecting to produce an audio frequency current. V V

4. The method of secret radio communication which comprises generating a lowsupersonic frequency Wave, Wobbling the-frequency of said Wave, transmitting said Wobbled low supersonic frequency Wave, producing an audio frequency current to be transmitted, modulatingvsaid Wobbled low supersonic frequency Waveby'said audio frequency current, suppressing the low supersonic frequency Wave and one of the side bands produced by said modulation, producing aharionic of said low Wobbled,supersonic frequency Wave and'providing thereby a Wobbled carrier frequency Wave, modulating said carrier frequency Wave by the remaining side band produced by said first mentioned mo dulation, suppressing the Wobbled carrier frequency Wave and one of the side bands produced by said last mentioned modulation, transmitting the remaining side band-produced by said modulation, receiving the loW Wobbled supersonic frequency Wave transmitted, producing from said Wobbled frequency Wave the harmonic which constitutes the Wobbled carrier frequency Wave, receiving the side i band transmitted, combining said side band and said Wobbled carrier frequency Wave and detecting to produce said low Wobbled supersonic frequency side band, combining'said lastmentioned side band With said transmitted low Wobbled supersonic frequency wave and detecting to produce said audio frequency current.

5. The method of secret radio communication which comprises generating a low supersonic frequency Wave, Wobbling the frequency of said wave, transmitting said obbled low supersonic frequency Wave, producing an audio frequency current to betransmitted, modulating said Web-bled loW supersonic frequency ave'by said audio frequency current, suppressing the low supersonic frequency Wave and one of the sidebands produced by said modulation, producing a harmonic of said lon Wobbled supersonic frequency wave and providing thereby a Wobbled carrier frequency Wave, modulating said carrier frequency Wave by the remaining side band produced by saidifirst mentioned modulation, suppressing the Wobbled carrier frequency Wave and one of the side bands produced by said last mentioned modulation, and

transmitting the remaining side band produced by said modulation. i V 1 6. The method of secret radio communication which comprises'generating a low supersonic frequency Wave, Wobbling the fre quenc-y'of said Wave, transmitting said Wobbled low supersonic frequency Wave, modulating said low supersonic frequency Wave by an audio frequency current, producing a harmonic of said 10W Wobbled supersonic frequency'wave and providing thereby a Wobbled carrier frequency Wave, modulating said carrier frequency wave byone side band produced by said first mentioned modulation,

quency Wave, and combining saidlast meny tioned" side band with said low wobb'led supersonic. frequency Wave to reproduce said audio frequency current. V e 7 The method of secret radio commumcation Which'comprisesgenerating alow supersonic frequency Wave,:wobbl1ng the frequency. of said Wave, transmitting said Woblating said lowfsupersonic frequency-Wave bled carrier frequency Wave, modulating said carrier frequency Wave by one side bandproduced by said first mentioned modulation,

and transmitting, one of the side bands produced by said last mentioned modulation.

8. The ;method of secret radio communication Which comprises generating a 10W supersonic frequency Wave, transmitting said -Wave, modulating said Wave by an audio frequency current producing a harmonic of said Wave and providing. thereby a carrier frequency Wave,

bled low supersonic frequency wave, modu-.

.by an :audio frequency current, producing a harmonic of said low Wobbled supersonic frequency'wave and providing thereby a Wobindependently modulating said carrier frequency ave-by onelof the side bands. produced by said first mentioned modulation, transmitting one of the sideban'dsproduced by, said last mentioned modulation, receiving the low supersonicv frequency Wave, produoing fromsaidwave the carrier frequency Wave, receiving the side band, combining said side bandwith said carrier frequency Wave,

and thus producing aside band of said low supersonic frequency Wave, v. and combining C said last IIIGIIlZIOIlGd side band withsald low supersonic frequency wave to reprodu ce said audio frequency current. 1

9. The process of secret radio communication vwhich comprises generating a low supersonic frequency ave, independently transmitting said wave, modulating said wave by an audio frequency current, producin a harmonic ofsaid wave and .pro- -v1d1ng therebywacarrier frequency wave,

modulating said. carrier frequency Wave by one of the side bands produced by saidfirsl V 'mentioned modulation, and transmitting one of the side bands produced by said last mentioned' modulation.

10. The methodof secret radioicommunication which comprises generating a wave of frequency B, wobbling said wave, by an amount W transmitting Bid-WV, producing an audio frequency current A, modulating B+VV by A and producing thereby B W-FA, B+VVA, and B'+WV, suppressing B A and B +'Vi. producing a harmonic of'Bi-VV, X(B+W), said last named wave to act as a carrier. frequency Wave, modulating X('B+VV) by B+VV+A, and producing (B+VV) I-A, (X1)V 1) (B +WV)'+A, receiving B+W and generating therefrom the harmonic X (B-l- WV), receiving X 1) (B +W) +A, and combining with X('B +VV) to produce B rtWV-l- A, and combining B +W+A with B +W to produceAp '7 11. The method of secret radio communication which comprises generating a wave of frequency B+W, transmitting said wave, generating an audio frequency current A,

,modu'lating B +WV by A and obtaining therefrom the waveB-l-W-l-A, producing a-harmonic of 'B+lV,- X(B-+W)', modulating X(B+W) by VBI-W+A and "obtaining therefrom (X+1-) (B +W) +A, transmitting the Wave (X t 1) B W) A, receiving the wave B +W, transforming the frequenc B +W into XtB'd WV), receiving (X+1' (B +A and combining with X(B+VV) to produce B+W+ A, combining saidlast mentioned wave with B+W and producing therefrom the current A. 7 V

12. The method of secret radio comunication which comprisesgenerating a wave of frequency B -l-VV, transmitting said Wave,

producing an audio frequency current A, modulating B +VV by A and obtaining therefrom B+VV+A, prod'ucingfrom: B-PW the harmonic 10 (Bi-W) modulating said harmonic wave'lO B-tW) by-Bl'-W+A and personic frequency wave by an audio frequency current, producing a harmonic of said low supersonic frequency wave and modulating said harmonic by'the product of the first mentioned modulation, transmitting one of the side bands produced by said last men tioned modulation, and independentlytrans mitting said low supersonic frequency wave,

and at a receiving station receiving'the side said low supersonic frequency wave and combining with said side band to produce a wave which will combine with said low supersonic frequency Wave to produce the audio frequency current.

14. Thecmethod of secret radio communication whi'ch comprises generating a low supersonic frequency wave, modulating said low supersonic frequency wave by an audio frequency current, producing a harmonic of said low supersonic frequency wave and modulating said harmonic bythe product of said' modulation, and transmitting one of the side bands produced by said second named modulation, transmitting independently the low supersonic frequency wave, and at a receiving station combining said transmitted waves to reproduce said audio frequency current.

15.Apparatus of the type described for secret radio communication which comprises *5 rent, push-pull means for modulating said supersonic frequency wave by said audio frequency current, means for transmitting said wobbled supersonic frequency wave, means for producing a harmonic of said wobbled supersonic frequency wave, means for filtering said last named wave, push-pull means for modulating said harmonic frequency wave by one of the side bands produced by said firstv mentioned modulation, means for transmitting one of the side bands produced by said last named modulation, means for receiving the wobbled supersonic frequency wave, means for amplifying said wave, means for producing a harmonic of said wave, means for filtering said last mentioned wave, means for receiving the side band transmitted, means for combining the side band with said harmonic frequenc wave, and producing therefrom a wave w ich when combined with said wobbled supersonic frequency wave will produce said audio frequency cur rent, means for combining said last named wavewith said wobbled supersonic frequency Wave to produce said audio frequency current.

16. Apparatus of the type described for secret radio communication which comprises means for generating a low supersonic frequency wave, means for wobbling the frequency of said wave, means for amplifying said wave, means for filtering said wave, means for provldlng an'audio frequency current, push-pull means for modulating said supersonic frequency wave by said audio frequency current, means for transmitting said means for generating a low supersonic fre quency Wave, means for Wobbling the frequency of said Wave, means for providing an audio frequency current, means for modulating said supersonic frequency Wave by said audio frequency current, means for transmitting said Wobbled supersonic frequency Wave, means for producing a harmonic of said Wobbled supersonic frequency Wave, means ror modulating said harmonic frequency Wave by one of the side bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving the wobbled supersonic frequency Wave, means for providing a harmonic of said Wave, means for receiving the side band transmitted, means for combining said side band Withsaid harmonic frequency Wave and producing therefrom a wave which, when combined With said Wobbled supersonic frequency Wave, Will produce said audio frequency current.

18. Apparatus for secret radio communication Which comprises means for generating a low supersonic frequency wave, means for Wobbling the frequency of said Wave, means for providing an audio frequency current, means for modulating said supersonic frequency Wave by said means for transmitting said wobbled supersonic frequency Wave, means for producing a harmonic of said Wobbled supersonic frequency Wave, means for modulating said harmonic frequency Wave by one of the side bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving the Wobbled supersonic frequency Wave.

19. Apparatus for secret radio communis cation Which comprises means for generating a low supersonic frequency Wave, means for modulating said supersonic frequency Wave by an audio frequency current, independent means for transmitting said supersonic frequency Wave, means for producing a harmonic of said supersonic frequency Wave,

" means for modulating said Wave by one of the side bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving. the supersonic frequency Wave, means for proaudio frequency current,

ducing a harmonic of said Wave, means for receiving the side band transmitted, means for combining said side band With said super? sonic frequency Wave to produce said audio frequency current. I

20. Apparatus for secret radio communication-vvhich comprises-means for generating a low supersonic frequency Wave, means for modulating said supersonic frequency Wave by an audio frequency current, independent means for, transmitting said supersonic frequency Wave, means for modulating a harmonic of said supersonic frequency Wave by one ofsaid side bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving the supersonic frequency Wave, meansfor receiving the side bands transmitted and means for combining said side band With said su )ersonic frequencywave to reproduce said audio frequency current. I I

21. Apparatus for secret radio communication which comprises means for generating a 10W supersonic frequency Wave, means modulating said supersonic frequency Wave by an audio frequency current, means for independently transmitting said supersonic frequency Wave, means for modulating a harmonicof said supersonic frequency Wave by one of the side bands produced by said first mentioned modulation 7 and means for transmitting one of the side bands produced by said last mentioned modulation. I

Apparatus of the type described which comprises means for generating a low supersonic frequency Wave, means for Wobbling the frequency of said Wave, means for modulating said Wobbled supersonic frequency wave by an audio frequency current,'means for transmitting said Wobbled supersonic frequency Wave, means for modulating a harmonic of said Wobhled supersonic frequency Wave by one of the side bands produced by said first mentionedmodulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving the supersonic frequency wave, means for receiving the side band transmitted, meansfor combming said side band With said supersonic frequency Wave to producesaid audio freuuency current. V

'23. Apparatusof the type described which comprises means'for generating a low supersonic frequency vvave means for Wobh'ling the frequency of said ave, means for modulating said Wobbled supersonic frequency;

Wave by an audio frequency current, means for transmitting said wobbled supersonic frequency wave, means for modulating 3,1131? 1 monic of said Wohbled supersonic frequency wave by one'of the side bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last'mentioned modulation. i

24;. Apparatus of the type described which comprises means for generating a low supersonicfrequency Wave, means for Wobbling the frequency ofsaid Wave, means for modulating said supersonic frequency Wave by an audio frequency current, means for transmitting said supersonic frequency Wave, means for modulating a harmonic of said supersonic frequency Wave by one of the frequency bands produced by said first mentioned modulation and means for transmitting one of the side bands produced by said last mentioned modulation. 7

25. Apparatus of the type described which comprises means for generating a low supersonicfrequency Wave, means for Wobbling the supersonicfrequency of said Wave, means for modulating said Wobbled supersonic frequency Wave by an audio frequency current, means for transmitting said Wobbled supersonic frequency Wave, means for modulating a harmonic of said Wobbled supersonic frequency Wave by one of the frequency bands produced by said first mentioned modulation, means for transmitting one of the side bands produced by said last mentioned modulation, means for receiving the Wobbled supersonic frequency Wave, means for receiving the side band transmitted, means for combining said side band With said Wobbled supersonic frequency Wave to reproduce said audio frequency current.

26. Apparatus for secret radio communication which comprises means for generating a low supersonic Wobbled frequency Wave, means for modulating said Wobbled supersonic frequency wave by an audio frequency current, means for transmitting said Wobbled supersonic frequency wave, means for modulating ahigh harmonic of said supersonic frequency Wave by one of the frequency bands produced by said first mentioned modulation,

means for transmitting the product of said last mentioned modulation, means for receiv ing the Wobbled supersonic frequency Wave, means for receiving the high frequency Wave transmitted, means for combining said Waves to reproduce said audio frequency current.

'27. Apparatus for secret radio communication Which comprises means for generating a low Wobbled supersonic frequency Wave, means for modulating said w'obbled supersonic frequency Wave by an audio'frequency current, means for transmitting said supen sonicfrequency Wave, means for modulating a harmonic of said Wobbled supersonic frequency Wave by one of the frequency bands produced by said first mentioned modulation, and means for transmitting the product of said last mentioned modulation.

28. Apparatus of the type described which comprises means for generating a 10W Wobbled supersonic frequency Wave, means for modulating said supersonic frequency Wave by an audio frequency current, means for transmitting said supersonic frequency Wave, means for modulating a high frequency Wave harmonic of said Wobbled supersonic frequency Wave by one of the frecuency bands produced by said first mentioned modulation, and means for transmitting the modulated high frequency Wave. a

29. Apparatus of the type described having its frequency periodically varied between certain limits which comprises means for producing a low supersonic frequency wave, means for modulating said supersonic frequency Wave by an audio frequency current, means for transmitting said supersonic frequency Wave, means for modulating a high frequency harmonic of said supersonic frequency Wave by one of the frequency. bands produced by said first mentioned modulation, means for transmitting the modulated high frequency wave, means for receivingthe low supersonic frequency Wave, means for receiving the highfrequency wave transmitted, means for combining said Waves to reproduce said audio frequency current.

30(Apparatus of the class described having its frequencyperiodically varied between certain limits which comprises means for generating a 10W supersonic frequency Wave, means for modulating said supersonic fre quency Wave by an audio frequency current, means for transmitting said supersonic frequency Wave, means for modulating a high frequency Wave harmonic of said supersonic frequency Wave, by one of the frequency bands produced by said first mentioned modulation and means for transmitting the modulated high frequency Wave.

JOHN HAYS HAMMOND, JR. 

